Indium-Gallium-Zinc Oxide Thin-Film Transistors for Active-Matrix Flat-Panel Displays

Amorphous oxide semiconductors (AOSs) including amorphous InGaZnO (a-IGZO) areexpected to be used as the thin-film semiconducting materials for TFTs in the next-generation ultra-high definition (UHD) active-matrix flat-panel displays (AM-FPDs). a-IGZO TFTs satisfy almost all the requirements for organic light-emitting-diode displays (OLEDs), large and fast liquid crystal displays (LCDs) as well as three-dimensional (3D) displays, which cannot be satisfied using conventional amorphous silicon (a-Si) or polysilicon (poly-Si) TFTs. In particular, a-IGZO TFTs satisfy two significant requirements of the backplane technology: high field-effect mobility and large-area uniformity.In this work, a robust process for fabrication of bottom-gate and top-gate a-IGZO TFTs is presented. An analytical drain current model for a-IGZO TFTs is proposed and its validation is demonstrated through experimental results. The instability mechanisms in a-IGZO TFTs under high current stress is investigated through low-frequency noise measurements. For the first time, the effect of engineered glass surface on the performance and reliability of bottom-gate a-IGZO TFTs is reported. The effect of source and drain metal contacts on electrical properties of a-IGZO TFTs including their effective channel lengths is studied. In particular, a-IGZO TFTs with Molybdenum versus Titanium source and drain electrodes are investigated. Finally, the potential of aluminum substrates for use in flexible display applications is demonstrated by fabrication of high performance a-IGZO TFTs on aluminum substrates and investigation of their stability under high current electrical stress as well as tensile and compressive strain

A study of polycrystalline MgZnO/ZnO thin-film transistor using the RF magnetron co-sputtering method

최근 ZnO 기반의 트랜지스터 연구가 활발히 진행이 되고 있다. 이와 같은 이유는 저온 성장된 박막임에도 불구하고 1 cm2/Vs 이상의 높은 이동도를 가지며, 가시광영역에서 높은 광 투과성을 가지기 때문에 고성능의 투명전자소자 개발을 가능하게 한다. ZnO 물질은 박막형 트랜지스터, 발광소자, 투명전극, 수광소자, 가스센서, 태양전지 등 다양한 분양에 적용 가능하다. 다양한 활용이 가능한 이유는 ZnO가 가지는 우수한 물성 때문이다. ZnO 는 3.37 eV 의 광밴드갭을 가지기 때문에 가시광영역에서 투명하다. 또한 Mg이 ZnO에 합금될 경우 광밴드갭이 늘어나기 때문에 UV 영역까지 발광소자 및 수광소자로써 활용이 가능하다. 또한 상온에서 60 meV의 여기자 결합에너지를 가지고 있으며, 직접천이형 밴드갭이기 때문에 발광소자에 큰 각광을 받는 물질이다. 또한 박막형 트랜지스터에도 많은 연구가 진행이 되고 있다. 박막형 트랜지스터는 기존의 비정질 실리콘 기반의 디스플레이의 백플레인 소자를 대체하기 위한 연구가 활발히 진행이 되고 있다. 평판형 디스플레이는 대면적화 고해상도, 뿐만 아니라 빠른 주파수를 요구함에 따라 기존의 비정질실리콘의 이동도로는 한계에 도달하게 되었다. 이를 해결하기 위해 LTPS (low-temperature polycrystalline silicon)이 필요로 하나 레이저 열처리등의 후속 공정이 필요로 하게 되어 공정단가가 상승하며, 대면적화에 문제가 제기되고 있기 때문이다. 하지만 ZnO 박막은 저온 성장한 박막임에도 불구하고 기존의 비정질 실리콘의 이동도를 넘으며, 공정단가 또한 LTPS 공정보다 저렴하며, 기존의 비정질 실리콘공정 기반시설을 이용할 수 있기 때문에 산화물 기반의 트랜지스터를 디스플레이의 백플레인용 소자를 개발하기 위해 많은 연구가 진행되고 있다. 이와 같은 ZnO 기반의 전자소자를 구현하기 위해서는 금속과 반도체 특성에 대한연구, 반도체 물질의 구조적 및 조성에 대한 연구, 또한 절연체에 대한 연구가 필요로 하다. 또한 ZnO 기반의 트랜지스터를 스위칭 소자로 활용하는 방법에는 MISFET (metal-insulator-semiconductor field effect transistor) 으로 구현 가능하다. 이와 같은 박막형 트랜지스터를 디스플레이의 백플레인 소자로 활용하기 위해서는 높은 이동도, 낮은 subthreshold swing, 해상력을 향상시키기 위한 높은 ON전류 및 전력소모를 줄이기 위한 낮은 OFF전류를 가지는 소자가 요구된다. MISFET 소자의 경우 절연체층이 필요로 하다. 절연체층의 특성요구 조건으로는 낮은 전압에서 작동가능하기 위해 높은 유전율 상수를 가져야 하며, 또한 낮은 누설전류를 가지며 피로파괴전압에 높은 저항성을 가지는 물질특성이 요구 된다. 또한 반도체에 캐리어를 주입하기 위해서는 오믹접합 특성이 필요로 하다. 반도체 층 물질로써는 ZnO에 Mg이 첨가될 경우 밴드갭 증가 및 자유전자의 캐리어를 감소시킨다. MgZnO의 경우 ZnO 기반의 산화물반도체에서 발생할 수 있는 산소공공에 의해 야기되는 소자의 불안정성을 해결할 수 있는데 이와 같은 이유는 Zn-O 결합 보다 Mg-O 결합에너지가 더 강하기 때문에 산소공공과 같은 결함 생성을 줄일 수 있다. 또한 ZnO 와 MgZnO 의 이종접합시에는 접합 경계면에 높은 전자 밀도를 형성이 가능하여 빠른 전자이동도를 얻을 수 있기 때문에 전자소자의 성능 향상을 가져 올 수 있다. 본 논문에서는 ZnO 와 MgxZn1-xO를 이용한 박막형 트랜지스터에 대해서 연구 하였다. Mg0.3Zn0.7O와 오믹접합을 얻기 Ni/Au 와 Ti/Au 물질을 사용하여 Mg0.3Zn0.7O 박막과 오믹접합을 확인하였으며 97.6 Ω·cm2 의 접촉비저항을 얻었다. ZnO 박막에 Mg 합금 비율이 1 과 10 at.% 박막을 증착하여 MgxZn1-xO 단일 채널을 가지는 박막형 트랜지스터를 제작하였으며, MgxZn1-xO-TFT 의 소자 평가가 이루어 졌다. Mg 첨가량이 증가함에 따라 MgxZn1-xO-TFT의 이동도는 감소하였으며, SS 값도 ZnO에 비해 증가하였다. ZnO와 MgxZn1-xO 이종접합 박막형 트랜지스터가 제작하였으며, ZnO 박막의 두께에 따른 소자 평가가 이루어졌다. MgxZn1-xO/ZnO TFT는 ZnO-TFT 보다 빠른 이동도를 가지며, 낮은 SS 값을 가지는 트랜지스터의 제작이 가능하였다. MgxZn1-xO/ZnO TFT에서 ZnO 박막의 두께는 ~10 nm 일 때 가장 우수한 특성을 보이는 것을 확인하였다. 또한 ZnO, MgxZn1-xO, MgxZn1-xO/ZnO TFT의 게이트 바이어스 스트레스 측정 및 히스테리시스 측정을 통해 소자의 신뢰성을 평가 하였다. MgxZn1-xO/ZnO TFT 이종접합으로 제작된 트랜지스터는 게이트 바이어스 스트레스 실험에서 ZnO 소자보다 낮은 전류 이동특성 및 낮은 히스테리시스 폭을 보이는 것을 확인하였다. 마지막으로 MgO박막을 절연체로 하는 ZnO 트랜지스터를 제작 하였다. MgO 경우 9.8의 유전체 상수를 가지기 때문에 저 전력의 소자로 기대할 수 있다. MgO 경우 증착 중 산소분위기 비율을 달리 하여 증착하였으며, 산소 분위기가 증가함에 따라 유전체 상수가 11.35 까지 증가하는 것을 확인 하였다.List of Tables List of Figures Abstract Chapter 1. Introduction 1.1 Oxide TFT application 1.2 Comparison of Oxide-based TFT with Si-based TFT 1.3 Metal oxide material 1.4 Physical properties of ZnO and MgO 1.5 Application of ZnO 1.6 Properties of MgZnO 1.7 Heterostructure of MgZnO/ZnO thin films 1.8 Evaluation of mobility for MISFET 1.9 TFT structures and princess Chapter 2. MgZnO and MgxZn1-xO/ZnO MISFET 2.1 Experiment method 2.1.1 Deposition of ZnO and MgxZn1-xO thin films 2.1.2 Fabrication of TFT devices 2.2 Results and discussion 2.2.1 Evaluation of MgZnO thin films 2.2.1.1 Structure properties 2.2.1.2 Optical properties 2.2.1.3 Electrical properties 2.2.2 Ohmic Contact of MgZnO 2.2.2.1 Motivation 2.2.2.2 Experimental detail 2.2.2.3 Results and Discussion 2.2.2.4 Conclusion 2.2.3 Single channel layer of MgZnO MISFET 2.2.3.1 Motivation 2.2.3.2 Experimental detail 2.2.3.3 Results and Discussion 2.2.3.4 Conclusion 2.2.4 Heterostructure of MgZnO/ZnO-MISFET 2.2.4.1 Motivation 2.2.4.2 Experimental detail 2.2.4.3 Results and Discussion 2.2.4.4 Conclusion 2.2.5 Stability properties of MgZnO TFT and MgxZn1-xO/ZnO TFT 2.2.5.1 Hysteresis properties 2.2.5.2 Positive gate bias stress Chapter 3. MgO Insulator 3.1 Study of TFTs by using a MgO insulator 3.1.1 Motivation 3.1.2 Experimental detail 3.1.3 Results and Discussion 3.1.4 Conclusion Chapter 4. Summary & Conclusion Referenc

Electronic and Optical Properties of Defects in Amorphous Oxide and Transition Metal Dichalcogenide Semiconductors

The optical and electronic properties of amorphous oxide thin films depend crucially on chemical composition, and deposition process variations which give rise to sub-gap defect states. Consequently, there is a need for a reliable, high-throughput method to extract sub-gap defect densities of states in amorphous oxide thin films. We present a novel in-situ method which uses ultrabroadband photoconduction (UBPC) measurements on TFT devices to extract the density of defect states in the sub-gap of a-IGZO. Both the TFT photoconduction response and geometric properties are used to isolate the absolute defect concentration in the sub-gap. The measured defect concentration increases from ~10^16 cm^-3 - 10^20 cm^-3 as laser photon energy is tuned from near conduction band state excitation to the valence band. The density of states (DoS) is calculated from the derivative of the defect concentration with respect to energy. The resulting fully-experimentally derived DoS reveals a series of broad defect peaks in the sub-gap and an exponential valance band Urbach tail. Density functional theory simulations classify the origin of the measured sub-gap density of states peaks as a series of donor-like oxygen vacancy states and acceptor-like metal vacancy states. Donor peaks are found both near the conduction band and deep in the sub-gap, with measured peak densities in the range of 10^17-10^18 cm^-3 eV^-1. Two deep acceptor-like metal vacancy peaks lie adjacent to the valance band Urbach tail region at 2.0 to 2.5 eV below the conduction band edge, with measured peak densities in the range of 10^18 cm^-3 eV^-1. By applying detailed charge balance, we show that increasing the density of metal vacancy deep acceptors shifts the a-IGZO TFT threshold voltage to more positive values. The DFT sub-gap identification is confirmed by showing the measured TFT electron capture times for metal vacancy acceptors are twice as long as that of oxygen vacancy donors. We found long recombination lifetimes of photoionized electrons into acceptor-like vacancies is one cause of TFT transfer curve hysteresis for photoexcitation wavelength, hv > 2.0 eV. To conclude the discussion of defects in a-IGZO, UBPC is used to directly measure the effect of hydrogen incorporated into a-IGZO TFTs. After hydrogen incorporation, oxygen vacancy DoS peaks are partially suppressed and the DoS near the valence band increases. This suggests that hydrogen hybridizes with oxygen vacancy sites resulting in metal-hydrogen (M-H) bonds, which form a new state at ~ 0.4 eV above the valence band. TFT transfer curves and spectrally resolved transient photocurrent lifetimes suggest that hydrogen can also replace acceptor-like metal vacancy states with donor-like O-H bonds. This had the effect of increasing the free electron concentration, decreasing the TFT threshold voltage by ~ 5 V and decreasing the long recombination time associated with metal-vacancy acceptor-like states by a factor of two. We then take up a discussion on the effects of defects on the photocarrier extraction efficiency and interlayer mobility in transition metal dichalcogenides (TMDs). By combining ultrafast photocurrent and transient absorption microscopy techniques, the complex dynamics of TMDs are distilled into a timeline of the efficiency-limiting steps. These steps are well described by a simple kinetic rate law that accounts for nonlinear exciton-exciton annihilation (~ 5 ps), linear depletion of carriers (~ 50 ps), and nonlinear defect-assisted Auger recombination (~ 1 ns) in WSe2. The rate law also predicts a nonlinear power dependence on incident photon-flux for transient absorption and AC-photocarrier extraction, which is verified in both WSe2 and MoS2. Experiments were performed on photodetectors made of few-layer 2D WSe2, which achieved both fast (down to ~ 80 ps) and efficient (up to ε ∼ 45%) photocurrent response despite defect-assisted carrier recombination competing with escape rates. Both the ultrafast photocurrent and transient absorption decay signals accelerated markedly due to a decrease in the electron escape time, τe, from 1.6 ns to 82 ps with increasing interlayer E-field. These escape rates suggest WSe2 has an interlayer electron (hole) mobility of only 0.129 (0.031) cm^2 V^-1 s^-1; nonetheless, efficient photocarrier extraction is still achieved as direct recombination becomes unlikely after electron-hole pairs are separated and localized on differing layers by the built-in or applied field. Spectrally resolved transient absorption and photocurrent each identify a photocarrier-trap-site with nanosecond lifetimes at ~ 1.25 eV below the conduction band, suggesting W-vacancies are the dominant sub-gap defects assisting in Auger-processes for incident photon fluxes as low as 10^12 photon cm^-2

Recent Advances in Thin Film Electronic Devices

This reprint is a collection of the papers from the Special Issue “Recent Advances in Thin Film Electronic Devices” in Micromachines. In this reprrint, 1 editorial and 11 original papers about recent advances in the research and development of thin film electronic devices are included. Specifically, three research fields are covered: device fundamentals (5 papers), fabrication processes (5 papers), and testing methods (1 paper). The experimental data, simulation results, and theoretical analysis presented in this reprint should benefit those researchers in flat panel displays, flat panel sensors, energy devices, memories, and so on

Amorphous Silicon Thin Film Transistor Models and Pixel Circuits for AMOLED Displays

Hydrogenated amorphous Silicon (a-Si:H) Thin Film Transistor (TFT) has many advantages and is one of the suitable choices to implement Active Matrix Organic Light-Emitting Diode (AMOLED) displays. However, the aging of a-Si:H TFT caused by electrical stress affects the stability of pixel performance. To solve this problem, following aspects are important: (1) compact device models and parameter extraction methods for TFT characterization and circuit simulation; (2) a method to simulate TFT aging by using circuit simulator so that its impact on circuit performance can be investigated by using circuit simulation; and (3) novel pixel circuits to compensate the impact of TFT aging on circuit performance. These challenges are addressed in this thesis. A compact device model to describe the static and dynamic behaviors of a-Si:H TFT is presented. Several improvements were made for better accuracy, scalability, and convergence of TFT model. New parameter extraction methods with improved accuracy and consistency were also developed. The improved compact TFT model and new parameter extraction methods are verified by measurement results. Threshold voltage shift (∆Vt) over stress time is the primary aging behavior of a-Si:H TFT under voltage stress. Circuit-level aging simulation is very useful in investigating and optimizing circuit stability. Therefore, a simulation method was developed for circuit-level ∆Vt simulation. Besides, a ∆Vt model which is compatible to circuit simulator was developed. The proposed method and model are verified by measurement results. A novel pixel circuit using a-Si:H TFTs was developed to improve the stability of OLED drive current over stress time. The ∆Vt of drive TFT caused by voltage stress is compensated by an incremental gate voltage generated by utilizing a ∆Vt-dependent charge transfer from drive TFT to a TFT-based Metal-Insulator-Semiconductor (MIS) capacitor. A second MIS capacitor is used to inject positive charge to the gate of drive TFT to improve OLED drive current. The effectiveness of the proposed pixel circuit is verified by simulation and measurement results. The proposed pixel circuit is also compared to several conventional pixel circuits.4 month

Amorphous Silicon Thin Film Transistor Models and Pixel Circuits for AMOLED Displays

Hydrogenated amorphous Silicon (a-Si:H) Thin Film Transistor (TFT) has many advantages and is one of the suitable choices to implement Active Matrix Organic Light-Emitting Diode (AMOLED) displays. However, the aging of a-Si:H TFT caused by electrical stress affects the stability of pixel performance. To solve this problem, following aspects are important: (1) compact device models and parameter extraction methods for TFT characterization and circuit simulation; (2) a method to simulate TFT aging by using circuit simulator so that its impact on circuit performance can be investigated by using circuit simulation; and (3) novel pixel circuits to compensate the impact of TFT aging on circuit performance. These challenges are addressed in this thesis. A compact device model to describe the static and dynamic behaviors of a-Si:H TFT is presented. Several improvements were made for better accuracy, scalability, and convergence of TFT model. New parameter extraction methods with improved accuracy and consistency were also developed. The improved compact TFT model and new parameter extraction methods are verified by measurement results. Threshold voltage shift (∆Vt) over stress time is the primary aging behavior of a-Si:H TFT under voltage stress. Circuit-level aging simulation is very useful in investigating and optimizing circuit stability. Therefore, a simulation method was developed for circuit-level ∆Vt simulation. Besides, a ∆Vt model which is compatible to circuit simulator was developed. The proposed method and model are verified by measurement results. A novel pixel circuit using a-Si:H TFTs was developed to improve the stability of OLED drive current over stress time. The ∆Vt of drive TFT caused by voltage stress is compensated by an incremental gate voltage generated by utilizing a ∆Vt-dependent charge transfer from drive TFT to a TFT-based Metal-Insulator-Semiconductor (MIS) capacitor. A second MIS capacitor is used to inject positive charge to the gate of drive TFT to improve OLED drive current. The effectiveness of the proposed pixel circuit is verified by simulation and measurement results. The proposed pixel circuit is also compared to several conventional pixel circuits.4 month